As a technique for enhancing image texture of a color image, photographed by a color image pickup device, such a technique has been proposed in which the color of a specific object in a color image, such as skin color, green of grasses or trees, or blue sky, is made to approach to a memory color of the object, thereby reproducing the desired color.
In Patent Document 1, for example, discloses such technique. In Patent Document 1 a representative color is extracted from an object region contained in the image. The representative color is compared with a preset center color of correction to determine an RGB correction parameter. In correcting the color of each pixel, the application strength of the correction parameter is controlled in dependence upon the distance of the color of the pixel from the center color. Specifically, the hue, saturation and luminance values are obtained on transformation from the RGB values that represent the color information of each pixel of a color image. The distance in the color space between the color of each pixel and the center color of correction is then calculated. The correction strength is then adjusted in dependence upon the distance calculated to emphatically correct the color of the object.
In this technique, color correction is made on the basis of addition/subtraction of the correction parameter in the RGB color space. For example, if the color is the skin color of a face of a human being, the RGB correction values are calculated on a per pixel basis, based on the distance between the RGB color and the center color of correction. If the face area in its entirety is to be bright, the correction parameter proportionate to a distance between the RGB color and the center color of correction is added to or subtracted from the RGB value of each pixel in the entire face area.
Patent Document 3 shows an apparatus and a method for color correction in which, in correcting the spectral colors of image data, the spectral colors are transformed into those in the color space of a lower dimension than the original dimension, so that color correction may be made in the color space of the lower dimension. The spectral colors of the proper order may thus be obtained in the lower diminution. In an arrangement of Patent Document 4, the spectral power distribution characteristic of the original reference white color is restored from the color temperature of the original reference white color which is the reference white color of the original color space. A spectral power distribution characteristic of a target reference white color is restored from the color temperature of the target reference white color which is the reference white color of the target color space. The surface reflectance of an optional color in the original color space is restored by exploiting tristimulus values of the optional color, the optical distribution characteristic of the original reference white color and the color matching function of the human being. The tristimulus values, representing the color in the target color space, are then found from the surface reflectance restored, the spectral power distribution characteristic of the target reference white color restored and from the color matching function of the human being. Patent Document 5 shows an arrangement in which a representative color of a specific object is extracted, and an optimum color correction parameter is set for the representative color extracted. In this manner, color correction may automatically be made on a crucial article in a natural image photographed under a variety of illumination environments. It is noted that the subject-matter of any of the Publications 3 to 5 is entirely different from that of the present invention as hereinafter described.
[Patent Document 1]
    JP Patent 3264273 (paragraphs 0036 to 0045)[Patent Document 2]    JP Patent Kokai Publication No. JP-P2003-317084A (paragraphs 0019 to 0021)[Patent Document 3]    JP Patent Kokai Publication No. JP-P2004-45189A[Patent Document 4]    JP Patent Kokai Publication No. JP-A-10-229499[Patent Document 5]    JP Patent Kokai Publication No. JP-P2001-92956A[Non-Patent Document 1]    T. HOSOI et al., ‘Face detection based on Generalized LVQ’, Shingaku-Gihou, The Institute of Electronics, Information and communication Engineers, 2003, Vol. 102, No. 651, pp. 47-52[Non-Patent Document 2]    Johji TAJIMA, ‘Image Engineering Series 10, Color Image Duplication, Fundamentals of Color Management’, Maruzen K K, Sep. 30, 1996, pp. 33-39